Power amplifiers for use with speech input signals have had the disadvantage that the output signal power has been only a small part of the power which is required to operate the amplifier. Switching amplifiers have been used to provide greater efficiency, but these amplifiers have not been suitable for battery operated equipment because they have required dual polarity power supplies. Further, these amplifiers have generally been complex and costly, partly because they required a large number of high speed switching devices. It has not been possible to provide satisfactory high speed switching devices, as required, by known integrated circuit techniques so that the resulting amplifiers have been large and not suitable for use in small portable equipment.
Known switching amplifiers have not been highly efficient when used to amplify speech signals where the average signal level is much below (12 db, for example) the peak level, as substantial power has been required to operate the amplifiers at low signal levels.